I'm running a sandbox with several adventure locations that have been visited, but not entirely cleared, and ones that have been left open for new residents to arrive.

I'm looking for a method for determining how these adventure locations change when no player character visits the place for some time.

After player characters have left a location, I restock it using wandering monsters and random treasure charts; this approach works fine.

How to handle longer PC absence, given that restocking only works once and only makes sense in locations with fairly vibrant ecology or factions that can grow or have reserves?

Ideally, the method would allow enough room for GM to consider the specific situation in their setting, would not require too much time or detailed attention to several locations, but would still be fairly rigid (e.g. "roll d6 per location, on a 1 make some changes" leaves too much to GM whim and provides too little inspiration).

Edit: There are several factions with clear goals, and several without clear goals, since they have not been seen in actual play too often. Ideally, the method would take decisions about which factions act, when and how successfully, out of my hands. I already have some of this in random encounter charts: One can encounter creatures from other nearby adventure locations in wilderness and in adventure locations, and these are easy to interpret as scouts, warbands, raiders, slavers, diplomats, or whatever.

8 Answers
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I tend to use something like Tarot cards for things like this. One to three cards for a location, major npcs or sometimes even player characters. One just to give a general feel of what might happen. Different decks normally have slightly different pictures. Sometimes looking at the card will give me inspiration, sometime the reading of a card. The suit of the Minor_Arcana can have many readings for example if you get Queen of Cups, may be a local Bishop comes to town; followed by 4 of Coins, he buys a local farm and Seven of Swords, but now he regrets his action (I wonder why?).

3) Some new, stronger faction moves in and wipes them out (or absorbs them to use as cannon fodder slaves) and alters the location to suit their tastes

4) Some new faction of approximately equal strength tries to move in; the next time the party comes by, the site is a warzone between the two sides

5) A schism appears in the lone faction, splitting it into two (or more) factions; apply one of the above options as appropriate

6) If they're beaten badly enough, they may simply die off; settlers could then move in and turn the area into a new town (assuming the location is appropriate for such); as an added bonus, if they know the PCs are the ones who cleared the place out, they may come to idolize the party

Obviously multi-faction locations are going to be more complicated. Generally it's easiest to look at who got hurt the worst when the PCs wandered through murdering everyone they saw, and adjust the inter-faction dynamics as appropriate.

Most adventure locations are not 'standalone' adventures, there should usually be a few links in each location that can help you determine how they evolve when the PCs are away. Some examples:

Your heroes only explored half of a dungeon before getting distracted. Were they sent there by an NPC who has an interest in the dungeon, and are there any local communities that might have a chance of discovering the now mostly-safe dungeon and taking the treasures for themselves?

Quests that have been forgotten about or never taken up in the first place (e.g. removing the kobolds from the sewers) may still evolve without the PCs being present (e.g. either the kobolds get out of hand and half of the town is burned to the ground, or some other adventurers solve the problem and become the new 'town heroes').

I use an online mind-mapper to lay out which NPCs/communities have interests with which adventure locations, and what groups of intelligent creatures are in tension with each other. This makes it very easy to see at a glance the possibilities of what might happen if a dungeon goes uncleansed, or quests unfulfilled.

Specifically, the ideas of generalizing your factions and random encounter tables into a "Random Faction Table", which includes an entry for 'Wandering Aventurers', and then another table which has outcomes/procedural-events for the situation (much like in Oblivious Sage's comment). In fact you could probably (meaning, I totally will) add some of those to the Conflict Outcome table for your setting.

I find that Justin Alexander's approach pulls together some very cool old school design trends that we tend to overlook now days, and I recommend reading his other articles, "ReRunning the Megadungeon" and "Wandering Adventures" as well, as they led up to this post.

(BTW, thanks to Pellanor for the link, I have been meaning to re-read the Project Slaughterhouse series and incorporate Justin's ideas, but had put that aside.)

Could you summarise the ideas that, for you, made this useful for this question?
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SevenSidedDie♦May 10 '12 at 18:04

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Having read that (massive!) article, I have to agree. I can see using it, with random monster tables and select location keys per zone, instead of 4e-style stat blocks. Very useful. (But this answer still needs editorial information before it earns my upvote—linking without explaining isn't enough to gain rep.)
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SevenSidedDie♦May 10 '12 at 20:19

One way to handle this is to think about who/what the in-game power players are in the areas around theses locations.

Could there be another orc tribe that once it realizes the destruction committed to it's neighbor tribe tries to take over their territory? Is there a local Prince that upon hearing that a keep on the northern road has been cleared sends a garrison of soldiers to hold the keep and start clearing the road? When a city's top elected official is revealed to be the vampire feeding on the city's young what happens to the city's politics--does violence start to break out?

PC's tend to make large changes to an area's power structure over a short period of time. This is bound to have repercussions.

Between sessions just give a little bit of though as to what NPCs might be playing in the sandbox with the PCs and what moves they would make.

My current method works as follows: Roll d6 for each adventure location left alone but that has been visited at least once by the player characters. On a roll of 2+, nothing significant has happened. On a roll of 1, roll d3.

On 2, consult a deck of Tarot cards. If there are several factions, consult a card for each major one.

On 3, roll a random encounter and change the location accordingly.

Options 1 and 2 leave enough room for interpretation to mirror recent events and tendencies. Also, I edit random encounter tables according to recent events, and so 3 also brings them to play automagically.

There's this monster and treasure making machine outside of reality that teleports random monsters and treasures in empty locations⸮ Maybe it's the True Form of Chaos (There Is No Hope At The Gate), maybe it's just the way the world is and why you need heroes to go and kill monsters. Just roll randomly, there's a zillion tables you can use.

If that does not work for you, what you may want is to have a story reason: Who could go there? Why would they go there? What drove them there? Why did they stay? Where did they accumulate treasure from? Who did they take it from? What is that treasure? Suddenly, you do not have "monsters" but interesting characters that the players can interact with. Ever saw the look on a Palandin's player who goes to kill the evil Orc just to see a kid run towards them with a rusty spoon shouting "You Leave My Mummy Alone!!!"...

Here's an example: The PCs find evidence there are orcs around. The PCs slaughter all the evil Orc at thier camp, leaving nothing alive or standing. They get loot. Good triumph over Evil. YAY! Well ... The orcs were there because of a deal made with $DemonLord and as long as the orcs were there worshipping, the $DemonLord slumbered. Now, $DemonLord is awake, pissed off, and planning to unleash a terrible plague (Hemorrhagic fever works well) on the land. He cursed all the orcs who became wraiths, forever trapped guarding irrelevant stones. New monsters: wraiths. New treasure: knowledge of why (and who) caused millions to die. Look on players faces when they realise it is their own fault: priceless.

The downvote was me. The question is asking for "a method for determining how these adventure locations change", which this answer doesn't provide. It provides "reasons for why the locations change". The question is looking for a system for determining changes, not justifications for changes. And if you meant this sort of creative brainstorming to be the method, it fails the questions requirement to be "fairly rigid" and doesn't avoid "leav[ing] too much to GM whim".
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SevenSidedDie♦Aug 29 '12 at 15:11